Prior art describes improved birefringent optical films with negative optical retardation dispersion which are suitable for use as achromatic optical retarders or compensators in liquid crystal (LC) displays. For example, WO 2008/119427 A1 describes a birefringent polymer film with negative optical dispersion, which is obtainable from a polymerisable LC material comprising as negative dispersion component compounds having the structure shown below or derivatives thereof:

The ratio of the negative dispersion component in the polymerisable LC material disclosed in WO 2008/119427 A1 is for example 50-60% of the total amount of solids (i.e. without solvents).
However, the bulky nature of the negative dispersion compounds according to the prior art are typically hard to align or give formulations with a narrow process window for annealing temperature, which is not convenient for mass production.
Moreover, the durability of the negative dispersion formulations according to the prior art is poor compared with standard positive dispersion formulations.
Thus, the negative dispersion materials disclosed in prior art still leave room for further improvements. For example, it is desired to reduce the amount of the usually costly negative dispersion compounds in the polymerisable mixture. Also, since the retardation of an optical retardation film is given by the product of its birefringence and film thickness, and thinner films are generally preferred in flat panel display applications, it is desired to increase the birefringence of the film so that the film thickness can be reduced while achieving the same retardation.
The films with negative dispersion can be used for example as A plate or C plate retarders. A positive A plate retarder is suitable for example for use in a circular polarizer, for example in an OLED panel to reduce reflections and therefore increase the contrast ratio. A positive C plate retarder is suitable for example for use as a component in an IPS mode LCD compensator film.
Also, the negative dispersion compounds and the polymerisable LC media comprising them, which are used for film preparation, should exhibit good thermal properties, in particular a modest melting point, a good solubility in the LC host and in organic solvents, and reasonable extrapolated clearing point, and should further exhibit excellent optical properties. Also, they should be suitable for making commercially viable quarter wave films in a large scale manufacturing process.
This invention has the aim of providing improved polymer films and polymerisable LC media for their preparation, which do not have the drawbacks of the prior art materials and have the advantages mentioned above and below.
Another aim of the invention is to extend the pool of polymer films and materials having negative dispersion that are available to the expert. Other aims are immediately evident to the expert from the following description.
It has been found that these aims can be achieved by providing polymerisable LC media and polymer films as claimed in the present invention.